The recent rise in popularity of high-performance mountain biking and off-road racing as serious professional and recreational sports have lead to challenging problems for bicycle manufacturers and designers. Metal fatigue, stress fracturing, and structural shattering are common consequences of severe impact forces encountered at high speeds on rough terrain. Strengthening impact resistance of rims by increasing wall thickness has the undesirable consequence of added weight. Attempts have been made to provide lightweight solutions using composite polymeric fiber-resin matrix wheels such as those disclosed in U.S. Pat. Nos. 5,540,485; 4,930,843; and, 5,246,275. These materials offer resilient properties, but create a difficult braking surface. Mountain-biking enthusiasts also complain that the wheels are not stable and require frequent adjustment to keep them in alignment. In addition, composite materials are expensive, production methods are complicated and generate toxic byproducts, both of which contribute to higher unit manufacturing costs.
In one alternative approach, Klein et al. (U.S. Pat. No. 5,499,864) disclose a truss type section geometry that reportedly attempts to maximize bending and torsional stiffness and strength while minimizing weight. Unfortunately, maximizing stiffness may decrease resilience and actually increase impact-related stress fracturing along angular planes within a rim profile. It would be highly desirable to have bicycle wheels with both increased stiffness and strength, but also some degree of resilience to improve impact resistance and decrease stress fracturing.
Aluminum metal matrix composite (MMC) are commercially available for extrusion use manufacturers of extrusion billets produced using powder metallurgy, spray and casting techniques. Ideally, to avoid interfacial failures between the matrix and particulate, MMC have an even distribution of fully wetted particles. Uniform aluminum MMC have reportedly be produced by mixing particulate suspensions of aluminum oxide (Al.sub.2 O.sub.3) into a low-silicon magnesium-aluminum alloy, e.g., 6061 or 2014, under controlled conditions including vacuum, e.g. a process such as that disclosed in U.S. Pat. No. 4,759,995. Addition of ceramic metal particles to aluminum alloys is reported to impart several different properties to the resultant MMC material: namely, decreased elongation (%), increased yield and ultimate strength (ksi) and Young's modulus stiffness (msi), but also with an increase in density (lb/in.sup.3). In use, however, properties of MMC are somewhat less predictable because they may change somewhat during extrusion, casting and forging with changes in microstructure, e.g., interfacial dynamics at particle-matrix boundaries, particle alignment, and/or settling of particles into gradients. MMC have recently found uses in non-aerospace applications including automobile drive shafts and brake rotors, and a bicycle frame marketed by Specialized.TM. as "M2.TM." is reportedly of an MMC aluminum alloy.
Objects of the invention provide high-impact-resistant bicycle wheels assembled from MMC rim extrusions having a rim profile capable of spreading compression loads in a manner designed to take advantage of the structural properties of metal alloy MMC materials.